Firefighters are monitoring a fire at the U.S. Polyco Plant after a "pop-off valve" blew off an asphalt tank Friday evening, according to police.

Officials said the fire is contained, but they are monitoring surrounding tanks. Ennis police said the large boom was heard at about 6:30 p.m. in the 3100 block of South Kaufman Street. The fire started when an asphalt rail container blew its pop off valve, causing the loud boom.

No injuries or evacuations are reported at this time. Workers in that area made it out safely. Officials said there are no safety or environmental issues as a result. Firefighters plan to allow the fire to burn itself out.

People near the area have been posting on Twitter, saying they felt a boom.

Waxahachie firefighters were also called to the scene.

Stay with WFAA as this story develops.